(1) Field of the Invention
The present invention relates to a cathode-ray tube, a cathode-ray tube apparatus, an image display apparatus, and a coil unit, specifically to a technology of correcting an image displayed on the cathode-ray tube having been affected by the terrestrial magnetism.
(2) Description of the Related Art
Color cathode-ray tube (CRT) apparatuses display images on the screen by allowing an electron gun to emit electron beams, which pass through passing holes of the color-selection electrode and reach the phosphor screen to which phosphors of red, green, and blue have been applied in advance at certain positions on which the electron beams are expected to land, allowing the phosphors at the landing positions to emit light of each color.
Here, an external magnetic field such as terrestrial magnetism (hereinafter, the external magnetic field is generically referred to as terrestrial magnetism) may act on the color CRT apparatuses to adversely affect the trajectory of the electron beams and inhibit the electron beams from landing on the phosphor screen at the expected positions. This is called “mislanding” or “landing deviation”, which results in color drifts or the like in the image displayed on the screen.
For the purpose of preventing the landing deviations, a magnetic shield is provided in the CRTs. However, it is difficult for the magnetic shield to completely remove the effects of the terrestrial magnetism in every direction.
In particular, it is difficult for the magnetic shield to remove a component of the terrestrial magnetism that is in parallel to the tube axial of the CRT (hereinafter, the component is referred to as tube axial component).
When affected by the tube axial component of the terrestrial magnetism, the electron beams land on the phosphor screen shifting in a clockwise or counterclockwise rotation about the tube axial at the areas around the perimeter of the screen, depending on the direction of the magnetic pole of the affecting tube axial component of the terrestrial magnetism. When this happens, the image on the screen viewed from outside appears to be tilting.
For this reason, color CRT apparatuses generally provide a function to adjust the image tilt.
More specifically, as shown in FIG. 1, a loop coil 307 is wound around a funnel 303 of a glass bulb 305 constituting a CRT 301, where the loop coil 307 lies in a plane β that is perpendicular to a tube axial Z of the glass bulb 305.
In the above construction, the value of the current that is fed through the loop coil 307 is adjusted using a variable resistor 309 connected to a direct voltage source 308, generating a magnetic field MC to cancel out a tube axial component MH of the terrestrial magnetism. With this operation, the image tilt is corrected. Note that in FIG. 1, reference number 302 represents a face panel, 304 a neck, and 306 a deflection yoke.
The above-described method, however, is not a perfect solution to the screen image deficiency. That is to say, the magnetic field MC generated by the loop coil and the tube axial component MH of the terrestrial magnetism are not equal to each other in an opposite direction. As a result, though it may correct the image tilt in a visual check, it does not completely correct the landing deviation on the screen at the areas around the perimeter of the screen. That is to say, the color drift, a problem to be solved in improving the image quality, remains unsolved.
Intending to solve the above-described problem, Japanese Patent Publication No. 6-69221 discloses a color CRT apparatus that simultaneously performs the landing deviation correction and the image tilt adjustment.
This technology corrects the image tilt and reduces the amount of landing deviation (that is to say, the amount of deviation of the electron beams, which have landed actually, from the positions on the screen at which the electron beams are expected to land when there is no effect by the terrestrial magnetism), by adjusting a position P of the loop coil 307 along the tube axial Z, which is originally disposed in a plane β that is perpendicular to the tube axial Z.
More specifically, the document discloses an observation result that when the position P of the loop coil 307 is shifted toward the deflection yoke 306 along the tube axial Z, the sensitivity of the tilt correction improves, and the amount of landing deviation correction decreases, and that on the contrary, when the position P of the loop coil 307 is shifted toward the face panel along the tube axial Z, the sensitivity of the tilt correction decreases, and the amount of landing deviation correction increases. Based on this observation result, the patent document proposes that an optimal position of the loop coil 307 is obtained in advance along the tube axial through experiments, and that the loop coil 307 is disposed at the obtained optimal position at which the amount of landing deviation becomes the smallest when the loop coil 307 receives a direct current as intense as decreases the image tilt to “0”.
The patent document states that it is possible to reduce the amount of landing deviation at the areas around the perimeter of the screen to a level that no color drift is observed, while correcting the image tilt.
However, there is a fear that this technology is incapable of coping with the high-definition and large-screen displays which are now in increasing demand. More specifically, with this technology, as the definition of the images on the color CRT apparatuses becomes higher or the screen becomes larger, the amount of landing deviation may increase to exceed the tolerance and the color drift may become more noticeable at the areas around the perimeter of the screen.